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Tuesday, April 26, 2011

Cross frame crank compatibility...

Being out of the bike industry insider club for several years, I missed the regular technical updates from Shimano and SRAM. The pages that make up these manuals are a bike designer's bible. When I designed the chainstays of the cross frame, I used what I had and thought I had all my bases covers so that a road compact crank would be compatible with the cross frame.

Well, I got it almost right. I physically checked the frame by installing compact cranks with external bearings from both SRAM and Shimano. External bearing cranks have no left/right adjustability with regards to chainline, arm clearance, ring clearance. Bearings go in. Cranks go on. Done. So I installed cranks from both Shimano and SRAM and thought - good, they fit.

Doh! I didn't check the new Shimano 105, Ultegra, and Dura Ace compact cranks. It seems their arm stance is narrower than their Tiagra compact or R600 compact cranks by enough that these cranks are not recommended on the Black Mountain Cycles cross frame.

Check out the chart below and notice the dimension W5. Here are some cranks and their respective W5 dimensions for reference.

In summary, Dura Ace, Ultergra, 105 compact cranks - not compatible with my cross frames. Any others are good to go. Sometimes compromises need to be made to fit those fat tires. I'm not sure about the compatibility of the previous generation of Dura Ace, Ultegra, 105 cranks, but will try to squeeze the info out of Shimano.

26 comments:

If you think YOU find the byzantine workings of the Shimano mindset a sometimes slippery slope, what do you think it is like for normal humans? In my painful struggle to attain enlightenment, however, I notice that SRAM seems to be more "user friendly" than their nefarious counterpart, leading me to wonder why they do not have a larger market share.

Have you ever installed a mountain double crank (28/40) on a cross frame with a road derailleur? The only reason I thought it may not work is the tail of the front derailleur may hit the chainstay because it is positioned so low.

Anon - that's something I'm going to look at soon. I have a SRAM 42/28 mtn crank that I want to check. Any mtn double will physically fit. The potential problem is if the bike is run with drop bars and brifters, you have to use a road front derailleur and I'm not 100% certain there is enough throw in the road front derailleur to get to to shift to the outer ring. It could be done using the mountain double front derailleur with bar-end shifters. I'm planning on testing some things and will report back when I can.

Regarding mtn doubles (26/38 or 28/40), the problem is that if you are running integrated road brake/shift levers, you need to run a road front derailleur (although, I have heard of adapters that allow using mtn f/d with road shifters). A road front derailleur won't make the throw to the much wider chainline of a mtn double.

If you run bar end shifters, there *should* be no problem using a mtn double specific derailleur. I say should because there may be an issue with the tail of the f/d cage hitting the chainstay as the bb drop is lower than on mtn bikes and the angle of the seat tube / chainstay is more acute.

Phil B: Middleburn with a 113 axle will give you a 50mm chainline - which is okay on a mountain bike with 135mm spacing and a mountain front derailleur that has more throw. However, road front derailleurs won't shift that far out - as you've found. A 108 will bring it in a bit and it might be just enough. I have found that Shimano's FD-R440 has a good amount of throw that will reach a 47.5mm chainline, but not enough for 50mm.

If you are using bar-end shifters, you could also use a mountain front derailleur that will work with either the 113mm or 108mm bottom bracket. The mountain front derailleur won't, however, work if you are using integrated brake/shift levers.